Emergency legislation to guarantee the use of anonymous evidence in court cases is to be published.

Justice Secretary Jack Straw will unveil a Bill due to be pushed through before MPs leave Westminster for their long summer break on July 22.The move follows a Law Lords ruling which questioned the use of anonymous witnesses, leading to the collapse of a £6 million murder trial at the Old Bailey.

Police feared other forthcoming trials would falter unless the Government moved quickly to clarify the law, and that a series of high-profile convictions would be quashed.

Mr Straw told MPs last week that urgent action was vital to prevent a "significant" number of cases being jeopardised.

He said: "Anonymous evidence is these days fundamental to the successful prosecution of a significant number of cases, some of which involve murder, blackmail, violent disorder and terrorism.

"Such cases could be jeopardised if we do not quickly fill the gap created by their Lordships' judgment."

The legislation will set out details of the process for allowing witnesses to remain anonymous and require that a judge "will have to be satisfied that the need for anonymity is satisfied, that a fair trial will be possible and that it is in the interests of justice".

Although the new law will be rushed through, it will be revisited as part of a package of legal reforms due to be introduced in the next parliamentary session, Mr Straw told MPs.

Police believe up to 60 people convicted of serious crimes in London alone could walk free if witnesses refused to reveal their names in a retrial.Lawyers for two of the four men found guilty of murdering Birmingham teenagers Charlene Ellis and Letisha Shakespeare in 2003 had already said they planned to appeal in the wake of the Law Lords' ruling.

Other high-profile convictions which it was feared could face challenges were the murders of Michael Dosumnu, Magda Pniewska, Toni-Ann Byfield and Zainab Kalkoh.

Dozens of trials across England and Wales have effectively been put on hold as prosecutors assessed the wide-ranging implications of the ruling.A Ministry of Justice spokesman declined to comment ahead of today's publication.